This document describes a rigorous-engineering fuel-consumption test procedure that utilizes industry accepted data collection and statistical analysis methods to determine the change in fuel consumption for individual trucks and buses with GVWR of more than 10000 pounds. The test procedure may be conducted on a test track or on a public road under controlled conditions and supported by extensive data collection and data analysis constraints. The on-road test procedure is offered as a lower cost alternative to on-track testing, but the user is cautioned that on-road test may result in lower resolution (or precision) data due to a lack of control over the test environment. Test results that do not rigorously follow the method described herein are not intended for public use and dissemination and shall not be represented as an SAE J1321-Type II test result. The previous SAE J1321 document lacked sufficient details related to between-run cool-down criteria, the use of volumetric flow meters to assess fuel consumption, and the reporting criteria for testing active aerodynamic technologies. Additionally, the previous version contained various formatting errors and a lack of references that support the proper use of the procedure. To address the limitations of the procedure, it was determined that a full revision of the document was required. An updated calculation spreadsheet is included with modification to the reporting precision of the final test results. There is no change to the data analysis process.